313D Psychrometrics and Humidity Control 2 — Questions and Answers
Question 1: What does the Sensible Heat Ratio (SHR) of a cooling system represent?
- The ratio of latent heat removal to total heat removal
- The ratio of sensible heat removal to total heat removal (sensible plus latent) (Correct answer)
- The ratio of supply air temperature to return air temperature
- The ratio of indoor humidity to outdoor humidity
Correct answer: The ratio of sensible heat removal to total heat removal (sensible plus latent)
SHR = sensible load ÷ (sensible load + latent load), indicating what fraction of total cooling capacity handles temperature reduction versus dehumidification.
Question 2: What happens to relative humidity when air temperature increases while the moisture content stays the same?
- Relative humidity increases because warmer air holds less moisture
- Relative humidity stays constant because no moisture was added or removed
- Relative humidity decreases because warmer air can hold more moisture (Correct answer)
- Relative humidity immediately reaches 100% at elevated temperatures
Correct answer: Relative humidity decreases because warmer air can hold more moisture
As temperature rises, the moisture-holding capacity of air increases, so with unchanged absolute moisture content the relative humidity (a ratio) decreases.
Question 3: What is the primary function of a humidistat in a residential HVAC system?
- To measure refrigerant pressure on the suction line
- To sense indoor relative humidity and signal the system to humidify or dehumidify (Correct answer)
- To regulate blower motor speed based on duct static pressure
- To measure the temperature differential across the evaporator coil
Correct answer: To sense indoor relative humidity and signal the system to humidify or dehumidify
A humidistat detects relative humidity and cycles humidification or dehumidification equipment to maintain the set humidity level, functioning like a thermostat but for moisture.
Question 4: How many grains of moisture are in one pound of water?
- 1,000 grains
- 5,040 grains
- 7,000 grains (Correct answer)
- 10,000 grains
Correct answer: 7,000 grains
There are exactly 7,000 grains per pound of water; HVAC technicians express air moisture content in grains per pound (GPP) of dry air.
Question 5: Under what condition does condensation form on interior window surfaces?
- Whenever indoor relative humidity exceeds 30%
- Whenever indoor relative humidity exceeds 50%
- When the window surface temperature drops below the dew point of the indoor air (Correct answer)
- Only when outdoor temperatures exceed indoor temperatures
Correct answer: When the window surface temperature drops below the dew point of the indoor air
Condensation forms on any surface whose temperature falls below the dew point of the surrounding air, regardless of a single fixed RH percentage.
Question 6: What does a high latent load in a residential building indicate?
- The building envelope has insufficient insulation for the climate
- There is significant moisture that the cooling system must condense and remove (Correct answer)
- The supply air temperature must be lowered to maintain comfort
- The ductwork is undersized and restricting airflow
Correct answer: There is significant moisture that the cooling system must condense and remove
A high latent load means large amounts of moisture — from occupants, infiltration, cooking, or plants — are entering the space and must be removed by the AC system.
Question 7: What is the bypass factor of a cooling coil?
- The ratio of air that bypasses the filter media to total airflow
- The fraction of airflow that passes through the coil without making effective thermal contact (Correct answer)
- The amount of refrigerant bypassed around the expansion valve at part load
- The heat transfer rate through the outer casing of the air handler
Correct answer: The fraction of airflow that passes through the coil without making effective thermal contact
Bypass factor is the fraction of air that passes through the coil without meaningfully contacting the coil surface, reducing cooling and dehumidification effectiveness.
313D Psychrometrics and Humidity Control 2
What does the Sensible Heat Ratio (SHR) of a cooling system represent?